The present invention relates to a sheet gripper for a sheet-displacing apparatus. More particularly this invention concerns such a gripper used in or on a roller of a printing or like machine.
A sheet carrier cylinder or roller is often provided with one or more sheet grippers normally constituted as a pair of gripper elements whose faces are engageable with each other to grip a sheet. It is essential in modern devices such as polychrome rotary printing presses that the sheets being printed pass from one roller to another without their predetermined positionings being changed. Thus the sheet gripper must not allow the sheet to slip at all relative to the element on which such a gripper is provided.
Such grippers, as mentioned above, are normally constituted as a pair of gripper elements at least one of which is movable toward and away from the other. The gripper elements can be bars or a single bar displaceable relative to a fixed substrate, or even a roller and a bar in combination.
In view of the increasing use of two-sided printing and polychrome printing, and the employment of coated papers, the importance of and difficulty of exactly holding a sheet has increased. It has been suggested to increase the gripping effect of a gripper by increasing the coefficient of surface friction of such an arrangement, or by shaping the face of at least one of the grippers. Such arrangements in practice have proven relatively expensive, and have often lead to embossing of the workpiece edge with the shape of the gripper element.